The present invention generally relates to an impedance matching circuit and a control method thereof. More specifically, the present invention is directed to an impedance matching circuit containing a power amplifying circuit for amplifying a modulation signal in a linear manner, and also to a method for controlling this impedance matching circuit.
Conventionally, this sort of impedance matching circuit is applied to a transmitter circuit for transmitting a transmit signal. This transmitter circuit includes a modulating circuit for modulating an input signal to output a modulation signal, a power amplifying circuit for power-amplifying this modulation signal in a linear manner to output a power-amplified modulation signal, and an antenna for transmitting this power-amplified modulation signal.
The input impedance of the antenna connected to the power amplifying circuit would be varied due to disturbances such as a change in ambient temperatures, an aging change, or direct touch of a human body to the antenna. As a result, electromagnetic waves (transmit signals) would be reflected at the input terminal of the antenna and then the reflection waves would be input to the power amplifying circuit. When high power reflection waves are input to the power amplifying circuit, there are some risks that this power amplifying circuit would be electrically destroyed.
To solve this problem, one solution has been proposed in, for instance, Japanese Laid-open Patent Application No. 60-145712 opened in 1985 as the control signal detecting circuit for protecting the power amplifier.
In this detecting circuit, the traveling wave and the reflection wave of the power-amplified modulation signal is sensed by the directional coupler provided between the power amplifier and the antenna. Then, the sensed traveling wave and reflection wave are detected by the respective detectors. Thus, two sets of power of the detected signals are compared with each other by the DC amplifier, so that the power ratio between them is outputted. The power ratio is inputted into the level adjusting unit and then, the gain of the power amplifier is so controlled as to be decreased when the ratio of the reflection wave's power value to the traveling wave's power value is larger than a preselected value. When the power value of the reflection wave is larger than this preselected value, since it is so controlled that the gain of the power amplifier is lowered, the power value of the reflection wave is also reduced, so that the electrical destruction of this power amplifier may be prevented.
However, although this detecting circuit may lower the gain of the power amplifier when the ratio of the reflection wave's power value to the traveling wave's power value is larger than a preselected value to thereby reduce the power value of the reflection wave, this detecting circuit can not suppress occurrences of the reflection waves per se.
Even if the power value of the reflection wave is low and this reflection wave is input to the power amplifying circuit for long time, this power amplifying circuit would be electrically destroyed.
In such a case that the linear power amplification by the power amplifying circuit is required, for instance, in the case that the AM (amplitude-modulated) signal is power-amplified, the reflection wave is inputted from the antenna, so that this AM signal could not be power-amplified in the linear region. When the AM signal is power-amplified by the power amplifying circuit in the nonlinear region, the resultant signal could not be correctly demodulated at the transmission counter party.
Furthermore, this detecting circuit has such a problem that since the gain of the power amplifier is reduced so as to lower the power of the reflection waves, the input signal could not be continuously power-amplified by the power amplifier at a constant gain.